The answer is given in the Vishnu Purana.
ParaBrahman creates through Lord Brahma, maintains through Lord Vishnu, and destroys through Lord Shiva.
srishtisthithyanthakaraneem brahmavishnushivaathmikaam sa sajnaam yaathi bhagavaan eka eva janaardhanaha
Translation:
“In order to handle creation, sustenance, and destruction, the one and only Janaardhana takes the names of Brahma, Vishnu, and Maheswara.” - Vishnu Purana 1.2.66
The term ‘Trimurti’ literally means ‘three forms’ and refers to them being the three guna-avataras (i.e., the three expansions that oversee the cycle of creation, maintenance, and destruction).
Hence, in this context, they are qualitatively nondifferent from one another.
śrī-bhagavān uvāca
ahaṁ brahmā ca śarvaś ca jagataḥ kāraṇaṁ param ātmeśvara upadraṣṭā svayan-dṛg aviśeṣaṇaḥTranslation:
“Lord Viṣṇu replied: Brahmā, Lord Śiva and I are the supreme cause of the material manifestation. I am the Supersoul, the self-sufficient witness. But impersonally there is no difference between Brahmā, Lord Śiva and Me.” - Srimad Bhagavatam 4.7.50
The reason the Trimurti exists is because it is believed that ParaBrahman never directly associates with the material realm to create, maintain, and destroy and acts indirectly through His expansions.
This is stated by Lord Brahma:
māyayāramamāṇasya na viyogas tayā saha
Translation:
“The Supreme Personality of Godhead never consorts with His illusory energy. Still, her connection is not entirely cut off from the Absolute Truth.” - Brahma Samhita 5.7
This is stated by Lord Vishnu:
ātma-māyāṁ samāviśya so ’haṁ guṇamayīṁ dvija sṛjan rakṣan haran viśvaṁ dadhre saṁjñāṁ kriyocitām
Translation:
“My dear Dakṣa Dvija, I am the original Personality of Godhead, but in order to create, maintain and annihilate this cosmic manifestation, I act through My material energy, and according to the different grades of activity, My representations are differently named.” - Srimad Bhagavatam 4.7.51
This is stated by Lord Shiva:
yam āhur asya sthiti-janma-saṁyamaṁ tribhir vihīnaṁ yam anantam ṛṣayaḥ
Translation:
“All the great sages accept the Lord as the source of creation, maintenance and destruction, although He actually has nothing to do with these activities. Therefore the Lord is called unlimited.” - Srimad Bhagavatam 5.17.21
Hare Krishna